A transformer circuit of that kind is for example known from the DE-OS No. 25 00 065. This circuit comprises a single setting unit with a transformer, the primary winding of which is fed by the supply voltage delivered from the voltage source. Provided at the secondary winding are several taps which can selectably be connected with the conductors leading to the load with the aid of automatically controllable switches. Hereby, it is made certain that the same alternating voltage amplitude is always conducted to the load even when the circuit is connected to voltage sources which in respect of the amplitude deliver different alternating voltages.
This known arrangement however displays a series of disadvantages. Thus, the entire power conducted to the load must be transmitted by way of the magnetic field of the transformer. The dimensioning of the transformer must therefore be adapted to this total load and correspondingly high losses result. If the load takes up a very high power, then the transformer must also be designed to be very large and beyond that cooled, which leads to appreciable manufacturing and operating costs. Beyond that, the known circuit is not suitable to switch over frequently and rapidly in order to keep the load voltage at least nearly constant in spite of corresponding changes in the amplitude of the supply voltage. If one were to operate the known arrangement in this manner, appreciable problems would also arise out of the fact that the entire power conducted to the load flows through the change-over switches. These switches would have to be operated under load on the one hand and special measures would have to be taken on the other hand in order to prevent that it comes to interruptions in the energy supply to the load during the switching-over.
Furthermore, voltage controlling transformer circuits are known which have at least one setting unit comprising two input terminals to which an alternating input voltage is applied, two output terminals delivering a controllable alternating output voltage, a transformer having a first winding which is connected to a first one of said two input terminals and a first one of said two output terminals, and a further winding the number of turns of which is greater than the number of turns of said first winding, a terminal connection conductor galvanically connecting the second one of said two input terminals with the second one of said output terminals, and switches by means of which at least one and usually several different control voltages can be applied to said at least one further winding so that in said first winding a voltage is induced which, in dependence on the winding sense of said further winding with respect to said first winding, is either additionally or subtractively imposed on the alternating input voltage of the at least one stage. As a consequence, the alternating output voltage of this stage is either equal to the sum of or equal to the difference between the alternating input voltage and the voltage induced in the first winding. This is based on the consideration that the required change in the amplitude of the supply voltage delivered from the voltage source in many cases of application amounts to only a comparatively small percentage, for example of .+-.25% of the amplitude. Therefore, the main part of the power is conducted to the load in galvanic manner by way of the first winding of the transformer, wherein due to the low number of turns of this winding and the low frequencies, at which high powers are delivered to loads, the inductance of this first winding produces only a very small voltage drop with correspondingly small losses.
By applying a control voltage U.sub.S to said further winding, the at least one setting unit of the transformer circuit can be brought into at least one switching state, in which a voltage .DELTA.U.sub.1 is induced in the first winding of the transformer, which voltage in accordance with the winding sense of the further winding with respect to the first winding is either added to or subtracted from the input voltage so that it applies for the output voltage U.sub.A varied relative to the input voltage: EQU U.sub.A =U.sub.E .+-..DELTA.U.sub.1 ( 1)
In that case, the relative magnitude of .DELTA.U.sub.1 with respect to the control voltage U.sub.S is given by the turns ratio w.sub.1 /w.sub.w of the first winding of the transformer to the further winding: EQU .DELTA.U.sub.1 =(w.sub.1 /w.sub.w).multidot.U.sub.S ( 2)
The turns ratio w.sub.1 /w.sub.w is here substantially smaller than 1 and lies in the range of 1:7 to 1:200. Beyond that, the current, which flows through the further winding in the first switching state, is to be so matched to the nominal load current, which flows through the first winding of the transformer, that the flux linkages of both windings are in terms of amount about equally great for a given turns ratio and display such an angular displacement each realtive to the other that the magnetic flux, which hereby results in the transformer core, leads to the desired induced additive or subtractive voltage drop .DELTA.U.sub.1 across the first winding of the transformer. It is evident that the induced voltage drop .DELTA.U.sub.1 on these presumptions is largely independent of the load current so that a constant difference between input voltage and output voltage of the setting unit can be maintained even when the load current fluctuates relative to its nominal value.
A substantial advantage of this arrangement is that merely the small part of the load, which is required for the induced amplitude change, passes by way of the magnetic coupling of the transformer. Thereby, the energy losses, which arise through the inductive energy transmission from one transformer winding to the other, are reduced to a quite appreciable degree. Thus, the transformer can be dimensioned to be correspondingly smaller and the effort, which is required for the cooling of the transformer, can be reduced. Only a small part of the total load also goes by way of the switches, with the aid of which the control voltage can be applied to the further winding of the transformer so that the switches are stressed far less even in the case of frequent switching operations. Beyond that, semi-conductor switches, for example triacs or switches being composed of V-MOS-transistors, which make possible an appreciably more rapid switching than mechanical switches, can be used even for very large loads. A complete interruption of the energy supply to the load during switching can in principle not arise, since the galvanic connection between load and voltage source remains maintained permanently by way of the first winding of the transformer.
If one applies no control voltage to the further winding of the transformer when the control unit is not disposed in the first switching state defined above, then--if further measures are not taken--the entire magnetisation of the transformer core is effected by the flux linkages of the first winding. This leads to the occurrence of a voltage drop across the first winding dependent on the magnitude of these flux linkages and thereby on the magnitude of the load current. This voltage drop, which occurs when the further winding is switched off, lets itself be used pin-pointedly as constant difference between alternating input voltage and alternating output voltage of the setting unit only in such cases of application, in which the load current is constant. Otherwise, this throttle effect of the first winding can be employed on the occurrence of a short-circuit at or in the load to limit the then flowing load short-circuit current to an uncritical amount.
For a universal usability of such a setting unit, it is expedient to take care that the magnetisation of the transformer core is not effected solely substantially by the flux linkages of the first winding also in the time spans, in which the setting unit is not disposed in the first switching state. This can for example take place through an auxiliary winding, which in the time spans, in which the further winding does not lie at a control voltage, is for example short-circuited with the aid of switches.
Through appropriate dimensioning of the number of turns and of the current which then flows through the auxiliary winding, the flux linkages of this auxiliary winding can be so set that no noteworthy induced voltage drop occurs across the first winding.
Thus, the output voltage of the setting unit is about equal to the input voltage in the time spans, in which the auxiliary winding is short-circuited. However, this equality can be attained only approximately and the apparatus effort required for this is comparatively great.
However, the at least one setting unit of the transformer circuit can be brought into different switching states by applying different control voltages to one or several further windings; this will be explained in the following for different embodiments:
When one designates the switching state, into which such a setting unit having one single further winding to which two control voltages may be applied is brought by applying a first control voltage U.sub.S1, as first switching state which is represented by above equations (1) and (2), then, by applying a second control voltage U.sub.S2 to the same further winding, under the same presumptions as above, a second switching state is obtained, in which a defined second voltage drop .DELTA.U.sub.2, which is largely independent of the load current, is induced at the first winding. In this case, it is valid for the output voltage U.sub.A : EQU U.sub.A =U.sub.E .+-..DELTA.U.sub.2 ( 3)
In that case, .DELTA.U.sub.2 likewise depends on the control voltage U.sub.S2 according to the above equation (2).
As control voltages the input voltage U.sub.E and the output voltage U.sub.A of the setting unit can be used, to which voltages the further winding is galvanically so connected directly with the aid of the switches while observing the winding sense that the one induced voltage .DELTA.U.sub.1 is added to the input voltage and the other induced voltage .DELTA.U.sub.2 is subtracted from the input voltage U.sub.E.
Thus, it is true for the output voltage U.sub.A in the first switching state EQU U.sub.A =U.sub.E +.DELTA.U.sub.1 ( 4)
and in the second switching state EQU U.sub.A.sbsb.2 =U.sub.E -.DELTA.U.sub.2 ( 5)
However, both these induceable voltages .DELTA.U.sub.1 and .DELTA.U.sub.2 can not be chosen each independently of the other. Rather, they are interlinked each with the other according to the equations EQU .DELTA.U.sub.1 =(w.sub.1 /w.sub.w).multidot.U.sub.E ( 6)
and ##EQU1## when w.sub.1 is the number of turns of the first winding and w.sub.w is the number of turns of the further winding of the transformer.
In order that also an unchanged transmission of the amplitude of the input voltage of the setting unit is possible to the output terminals of the setting unit, the setting unit can furthermore be brought into a third switching state, in which no voltage is induced in the first winding of the transformer. In order that the first winding in this third switching state does not develop any choke effect with a correspondingly high voltage drop, care must in that case be taken that the magnetisation of the transformer core is not effected substantially through the flux linkages of the first winding alone.
This can be done in different ways as will be explained in detail below. The only important fact is, that in this third switching state, only an extremely small voltage drops across the first winding of the transformer so that the output voltage of the setting unit is to a good approximation equal to the input voltage: EQU U.sub.A.sbsb.3 =U.sub.E ( 8)
A first way for realizing the third switching state is to provide a switch, with the aid of which the further winding can be short-circuited, while it is at the same time separated from all control voltages.
Because of the small voltage drop across the first winding, only a small voltage is also induced in the further winding so that the short-circuit current flowing in the current circuit of the further winding remains small and causes only very small power losses.
In order not to overload the transformer it must be guaranteed that the short-circuit switch is closed only whilst the switches, which serve for applying a control voltage, are open. Also care must be taken that the switches which serve for applying the one control voltage are closed only, whilst the switches which serve for applying the other control voltage are open and vice versa.
In order to make a simultaneous closing of these switches impossible, the switching state of each switch can be monitored with the aid of an associated sensor unit and a closing command for a previously open switch can be suppressed by a blocking circuit when the output signal of the sensor unit of the other switch indicates that one of these other switches is still closed.
It is desirable that the output voltage U.sub.A of the setting unit during the switching-over from one switching state into the other passes as rapidly and as "smoothly" as possible, i.e. without strong upward or downward fluctuations of the absolute amplitude amount of the alternating output voltage, from its old to the new amplitude value. However, with an embodiment in which the third switching state is obtained by short-circuiting the further winding, this cannot be done in an optical way since for the closing and opening of the switches certain switching criteria must be observed, which make it impossible to switch over from one amplitude value of the output voltage to another so rapidly that the new amplitude value is attained stably after less than a full oscillation period of the alternating load voltage.
Basically it is possible to obtain the third switching state by electrically connecting in parallel the further winding of the transformer with the first winding.
Thereby, one obtains a short-circuited transformer in this third switching state with two windings, which are wound in parallel opposition on the core of the transformer and lie electrically parallel each to the other at the same voltage. The currents, which in that case flow in both the parallelly opposed windings, each try to build up a magnetic field in the core of the transformer; these fields are however directed each against the other and substantially cancel mutually. The stray inductance and the ohmic resistance of the first winding flowed through by the load current are very small. Thereby, the voltage drop arising across it is very small and above equation (8) applies to a good approximation.
The current flowing through the further winding is also correspondingly small, since the further winding possesses a substantially greater impedance than the first winding of the transformer. Hereby, the load current thus flows practically exclusively through this first winding.
In principle, four switches suffice for a transformer which possesses only a single further winding in order to be able to bring the setting unit concerned into the named three different switching states.
If no further measures are taken, it must in this case also be observed with care that the input voltage and/or the output voltage of the setting unit is not short-circuited by simultaneous closure of corresponding switches, causing the flow of an impermissibly high short-circuit current. This however means that certain switching criteria are to be observed for the opening and closing of the switches, which criteria delay the attainment of the new amplitude value on the transition from one switching state into another.